The solar cell is spreading rapidly as a clean energy source replacing fossil fuels. At present, majority of the solar cell for practical use is the solar cell using the Si bulk crystal, particularly the market share of the Si bulk polycrystal is overwhelmingly high. The present invention relates to production of the Si bulk polycrystal ingot of high quality and a high yield by upgrading the total ingot using a practical method for producing according to a cast growth method used most generally with respect to the Si bulk polycrystal which has been put into use most and has secured a biggest share in the solar cell market.
In order to really deploy most safe and environment-friendly solar cell of the Si-based bulk polycrystal on a global scale, technological development enabling the low cost and high yield production of highly efficient solar cell using safe Si resources which are present richly is required. At present in the country and overseas, the main stream of the practical technology is the method for producing a solar cell by growing a large volume Si bulk polycrystal by a cast method using unidirectional growth from a Si melt and cutting out into a thin sheet wafer. However, the biggest problem of the Si bulk polycrystal grown by the unidirectional growth cast method which is an ordinary cast growth method is that many small grains are formed in the early stage of the growth, the crystallographic orientation of these grains is random, the ratio of the crystal boundaries which are the random grain boundaries is high, and the Si bulk polycrystal with high quality cannot be secured over the entire ingot.
These random grain boundaries affect the solar cell performance adversely and are present over the entire Si bulk polycrystal ingot. Due to these causes, in the Si bulk polycrystal ingot produced by the ordinary cast growth method, the yield of the ingot which is the ratio usable as a solar cell as a result is dropped down to approximately 60%.
Also, because the orientation distribution of the crystal grains of the Si bulk polycrystal produced by an ordinary cast growth method is random, it is difficult to make an excellent surface texture structure for effectively utilizing the sun light by making the sun light reflected at the solar cell surface incident on the solar cell again. Further, it has become a problem that energy conversion efficiency lowers because of the causes such that the impurities such as iron introduced to the inside of the crystal due to the production method become the recombination center of the photo-generated carriers. In addition, the dislocation present in the vicinity of the grain boundary and inside the grain also becomes a serious defect to act as a recombination center with high recombination velocity.
The present invention relates to a production technology of a Si bulk polycrystal ingot controlling the grain orientation and the grain size in a Si bulk polycrystal ingot, capable of inhibiting generation of a crystal defect such as the random grain boundary and dislocation, and capable of securing a high yield ingot without deteriorating the crystal quality. It is an important invention enabling to produce the Si bulk polycrystal ingot for a solar cell with high quality and high homogeneity (high yield), to deploy the solar cell widely to the world, and to promote to solve the energy and environment problems. Also, when the Si bulk polycrystal ingot produced by the production technology of the present invention is used, the surface texture structure is easily made, and a highly efficient solar cell can be produced due to this point also.
The Si bulk polycrystal ingot for a solar cell is produced by a cast growth method. In the cast growth method, the Si bulk polycrystal ingot is produced by solidifying a Si melt in one direction from the lower part of a crucible upwardly using the Si melt poured into the crucible which is coated with a release agent or the Si melt molten within the crucible made of the quartz which is coated with the release agent and the like.
In the Si bulk polycrystal ingot produced by an ordinary cast growth method, the grain size and the grain orientation are random because of a number of random crystal nuclei formed in the bottom face of the crucible in the early stage of the growth, majority of the grain boundaries are the random grain boundaries, a number of dislocations, sub-grain boundaries which are a kind of dislocation and impurities are present, and it is difficult to produce the Si bulk polycrystal ingot which is of high quality over the entire ingot. Also, owing to accumulation of detailed studies by the inventors, it has been clarified that the stress applied by growing of the crystal contacting the crucible is one of the causes of occurrence of the dislocation. Contact of the ingot crystal and the crucible becomes a cause of diffusion of the impurities such as iron contained in the release agent into the crystal. Therefore, in order to produce the Si bulk polycrystal ingot with high quality and high homogeneity with a high yield, it is indispensable to make the Si bulk polycrystal ingot grow not contacting the bottom face of the crucible and the side face of the crucible as much as possible.
As a publicly known technology for making the crystal grain orientation uniform, there is a method of producing a Si bulk polycrystal ingot for a solar cell with uniform orientation by arranging Si single crystals on the bottom face of a crucible and pouring a Si melt thereon (refer to Patent Document 1, for example). However, this method is not practicable from the viewpoint of the production cost because the Si single crystals are utilized as seed crystals. Also, because the Si bulk polycrystal ingot grows contacting the crucible, high quality ingot cannot be obtained.
As a publicly known technology for making the grain orientation uniform, there is a method of producing a Si bulk polycrystal ingot with uniform crystal grain orientation by making dendrite crystals with uniform growth orientation develop at the bottom face of a crucible containing a Si melt and making it unidirectionally grow on the upper face of the dendrite crystals (refer to Patent Document 2, for example). However, although this method is useful as a method to make the grain orientation uniform, it cannot prevent the contact with the crucible.
As a publicly known technology for making a Si bulk polycrystal ingot glow without contacting a crucible, there is a procedure that a melt of a semiconductor is maintained at a temperature near the melting point in a crucible, then cooling gas is blown to the melt or cooling gas is blown and a crystal and a metal bar are immersed for controlling nucleation, thereafter solidification growth is started for poly-crystallization (refer to Patent Document 3, for example). However, in this method, a method for uniformizing to a specific crystal orientation and a control procedure of a grain size are not disclosed.
As a production method for a Si bulk polycrystal ingot with uniform grain orientation in which the crystal in the middle of the growth does not contact a crucible, there is a method for growing a Si crystal by a Czochralski method with three kinds of single crystals controlled so as to include two Σ=3 grain boundaries and one Σ=9 grain boundary with a {110} plane as a growth face as a seed crystal (refer to Non-patent Document 1, for example). However, this method is not practical from the viewpoint of the production cost because an expensive Si single crystal is utilized as a seed crystal and is not suitable as a practical technology for producing a large ingot also.    Patent Document 1: Japanese Unexamined Patent Application Publication No. H10-194718    Patent Document 2: International Application Publication No. 2007/063637 pamphlet    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2007-45640    Non-patent Document 1: G. Martinelli and R. Kibizov, “Growth of stable dislocation-free 3-grain silicon ingots for thinner slicing”, Appl. Phys. Lett., 1993, 62, p. 3262